An object can be levitated within a rectangular chamber by propagating acoustic waves that set up a standing wave pattern, so the object is held at a pressure node (location where pressure is a minimum). The object can be stably held along three dimensions by establishing three mutually perpendicular standing wave patterns, the object gravitating to a point where pressure nodes of the three patterns intersect. Where the same wavelength is propagated along two or three dimensions (with corresponding finite air particle velocity components at the object position to apply a torque to the object), the object undergoes rotation in an amount dependent upon the phase difference between the waves of the same wavelength. U.S. Pat. No. 3,882,732 by Wang et al., shows (in his FIG. 13) a chamber of square cross section with equal wavelengths along two directions, which rotates the object by controlling the relative phases of waves of the same wavelength.
While the use of a square chamber and equal wavelengths along two perpendicular directions to levitate and rotate an object, provides a simple and effective technique in some applications, its use can be limited. In high temperature environments, as where the levitated object is heated to process it, temperature gradients along the path of the acoustic waves, between the room temperature of an acoustic drive and the high temperature chamber, can produce significant phase changes in the degenerate wavelengths that results in uncontrollable rotation. Another cause of uncontrolled rotation is partial coupling of acoustic waves of the same wavelengths initially propagated in different directions, which can modify their phase relationship and lead to unanticipated object rotation. Such rotation is especially troublesome where very high sound intensities are utilized for levitation so a slight phase difference may produce a large object-rotating torque.